SystemUI: allow update Mobile Network Type tile display

For the moment, this tile is only updated when clicking on it.
With these changes we have the same behavior as the other tiles:
if setting is changed in system settings, Mobile Network Type tile
display is updated.

    * Patch Set 1: Initial commit
    * Patch Set 2: Some corrections, commit message changed

Change-Id: I6cadadf4b9e3c9f478a4286e26d657da4f3ff015